It is widely acknowledged that there is an acute, worldwide shortage of human organs for transplantation. This is in spite of legislative changes and education programs to increase public awareness of the problem. In the United States, for example, there is an estimated annual shortfall of approximately 18,000 kidneys/year. Similarly, in Australia in 1992, only 41% of renal patients awaiting transplantation received transplants. In Japan the imbalance between supply and demand is even greater due to religious prohibitions on the use of organs from cadaveric donors.
The benefits of transplantation can be seen by comparing the rehabilitation rates of transplant patients with those of dialysis patients. In Australia and New Zealand, the majority of transplant patients (60%) are capable of full time work or school with a further 10% in part time work, while only 7% are unfit for work. In contrast, 23% of dialysis patients are capable of full time work or school, with 15% involved in part time work and 20% unfit for work. The remainder are “retired.” Fifteenth Report of the Australia and New Zealand Dialysis and Transplant Registry (ANZDATA), Queen Elizabeth Hospital, Woodville, S.A., APS Disney, ed. (1992).
The direct financial cost of dialysis in Australia and New Zealand is approximately $A45,000/patient/year. In addition, indirect costs due to unemployment and sickness are higher in dialysis patients and the social costs are considerable. Transplantation engenders an expense of approximately $A30,000/patient in the first year and $A14,000/patient/year thereafter. These statistics indicate that a) transplantation is the optimal therapy for end stage renal failure; b) there is an undersupply of donor kidneys; and c) present strategies aimed at increasing the transplant rate have been less than successful. There are, in addition, serious shortages of other transplantable organs including hearts, livers, lungs and pancreases.
The use of xenografts (transplants between species) is one option for overcoming the short supply of human organs for transplantation. Non-viable, non-antigenic xenografts are commonly used in vascular reconstruction (bovine arteries) and in cardiac surgery (porcine cardiac valves). However, despite their occasional use in the past, immunological barriers have prevented the common use of viable xenografts. Between 1964 and 1991 a total of 27 non-human primate to human organ xenografts was reported; the longest reported patient survival was 9 months. Two liver transplants from baboon to human were recently performed in anticipation that modern immunosuppressive therapies could cope with the severe rejection problems likely to occur in xenotransplantation. To date, the course of one of these patients has been reported, and in this case rejection was not the direct cause of death. Starzl et al., Baboon-to-Human Liver Transplantation. Lancet 341: 65–71 (1993). This clinical experience indicates that a) non-human organs can function and support human life; b) rejection episodes can be reversed by conventional anti-rejection therapy; and c) the mechanisms of rejection are similar, in principle, to those in allograft rejection.
It is unlikely that primates will be a satisfactory source of organs for xenotransplantation. Most are endangered species, breed slowly in the wild and poorly in captivity. The baboon is an exception to these generalizations, but other disadvantages limit the usefulness of this species. Baboons have single pregnancies, long gestation times, are difficult and expensive to maintain and may be infected with or carry organisms, particularly viruses, that are pathogenic in humans. For hearts and kidneys where organ size may be a consideration, the smaller primates are unsatisfactory as donors to human adults. Finally, the use of primates is likely to arouse considerable opposition from the public.
These difficulties have led to renewed interest in the use of non-primate species as organ donors for human patients. The pig is a widely acknowledged choice for xenotransplantation into humans. The pig erythrocyte diameter (6.5 μm) and, by implication, its capillary size, are similar to humans, facilitating connection of xenografts to the human circulatory system. The pig breeds well in captivity, has a short gestation time and produces large litters. In addition, pigs can be bred and maintained in low pathogen facilities, can be reared to any size and do not arouse the level of public reaction associated with primates.
The immunological barriers to use of pig organs in human patients include a) an immediate severe (“hyperacute”) rejection phenomenon that develops in minutes to hours after transplantation, and b) a proposed acute rejection that develops in days to weeks. Once the hyperacute rejection phenomenon has been overcome, it is expected that normal acute rejection would ensue. This form of rejection is thought to be similar to that experienced with allografts (transplants between individuals of the same species) and should be amenable to normal immunosuppressive therapies.
Both preformed “natural antibodies” (xenoantibodies) and complement regulating factors in human serum are thought to be involved in the process of hyperacute rejection. Hyperacute rejection is thought to be initiated when xenoantibodies bind to epitopes on the endothelium of a donor organ, activating the classical complement pathway.